Allocating shared memory in Postgres

Hi,

I need to allocate shared memory in Postgres 11.0 outside the
initialization phase. In order to achieve this I have done the following:

- increased the amount of shared memory by increasing the value of size in
CreateSharedMemoryAndSemaphores (int port) in ipci.c. I have made sure that
the amount of memory that I need, say m << M, where M is the amount of
additional shared memory that I have allocated.

- during a particular query, where I need to allocate shared memory (which
is a function of the sizes of the tables in the query), I invoke
ShmemInitStruct() in shmem.c

This seems to work in my case, although I haven't tested it extensively.

My concern here is that when I go through the NOTES in shmem.c, I find the
following line:

Fixed-size structures contain things like global variables for a module and
should never be allocated after the shared memory initialization phase.

I'm allocating a shared array data structure through ShmemInitStruct and
I'm not sure if the lines above apply to my case, since I'm doing the
allocation during a query.

Any help/clarifications in this regard would be appreciated.

Best,
-SB